I. The Problem with Sub-Optimal Use of Phosphatic Fertilizers.
Phosphorus (P) is an important nutrient required for plant growth and development, making up 0.2% of plants on dry weight basis. (Vance, 2001; Sachachtman, et al., 1998.) It is a significant part of a plant's physiological and biochemical activities, such as photosynthesis, carbon metabolism, membrane formation, energy generation, nucleic acid synthesis, glycolysis, respiration, enzyme activation/inactivation and nitrogen fixation. (Bucio, et al., 2003.)
Plants absorb most of their phosphorus as the primary orthophosphate ion (H2PO4−2) and a smaller amount from the secondary orthophosphate ion (HPO42−). Plants also absorb certain soluble organic phosphates (i.e. PO43− in nucleic acid and phytic acid). (Sekhar and Aery, 2001; Mullins, 2009.)
Amongst a number of advantages, the addition of phosphorus creates deeper and more abundant plant roots. (Gupta and Sen, 2012.) Conversely, a phosphorus deficiency alters root architecture, which ultimately affects seed development and normal crop maturity. (Williamson, et al., 2001.) Thus, the availability of adequate phosphorus is fundamental to stimulate early plant growth and hasten maturity.
However, phosphorus is among the least bio-available nutrients in soil (Takahashi and Anwar, 2007.) While the total amount of phosphorus is 0.05% of soil content on average, only 0.1% of that amount is available to plants. (Zou, et al., 1992.) Even in fertile soils less than 10 μM is available at pH 6.5 where it is the most soluble. (Gyaneshwar, et al., 1998.)
Soil phosphorus is found in both organic and mineral pools. Twenty percent (20%) to eighty percent (80%) of soil phosphorus is found in an organic form, such as phytic acid, while the rest of phosphorus is found as inorganic fraction. (Sachachtman, et al., 1998.) Because most arid and semi-arid soil regimes are deficient in phosphorus, phosphatic fertilizers are required to replenish the phosphorus demanded by crop plants.
However, more than 80% of the phosphorus applied as fertilizer stagnates in an immobile pool due to the presence of iron (Fe) and aluminum (Al) in acidic soils, and calcium (Ca) in neutral and alkaline soils, resulting in insolubilization, precipitation, and adsorption or conversion into an organic form through biological activities. (Harris, et al., 2006.) This so called “fixation” of externally supplied phosphorus is common in arid alkaline and calcareous soils because of the higher activity of the calcium. And when coupled with a high pH typically found in soils, the application of externally supplied phosphorus favors the precipitation of relatively insoluble di-calcium phosphate and other basic calcium phosphates such as hydroxyl-apatite and carbonato-apatite decreasing the activity of phosphorus. Research suggests that phosphorus “fixation” in alkaline soils is largely attributed to the retention by clays saturated with calcium. Because calcium ions can retain greater amount of phosphorus than those saturated with sodium or other mono-violent ions, the formation of clay (Ca2+H2PO42−) is believed to be the most likely culprit.
Low availability of phosphorus to crop plants is a worldwide problem and thus crop yield on 30 to 40% of world's arable land is limited by phosphorus availability. (Vance, et al., 2003.)
To overcome the consistent deficiency of phosphorus in alkaline/calcareous soils, soluble phosphatic fertilizers are applied to agricultural fields to maximize crop production. (Shenoy and Kalagudi, 2005.) Commercial fertilizers have played a very significant role in enhancing the per acre yield of crops and in return feeding the rising population of the world.
Unfortunately, however, current production of phosphorus fertilizers is insufficient to meet rising demand. The alarming depletion of world resources of rock phosphate, along with their low use efficiency, has resulted in consistently increasing prices of phosphorus fertilizers. Elevated fertilizer prices, their scarcity at the right time of application, as well as sub-optimal doses largely accounts for low phosphorus fertilizers use. (Alam, et al., 2005.) In addition to these problems, the fixation/precipitation/adsorption/transformation of phosphorus decreases efficiency of applied chemical phosphorus fertilizers. Such sub-optimal use of phosphate fertilizers has led to exogenous application of substantial quantities of phosphatic fertilizers in agricultural fields. (Vassilev and Vassileva, 2003; Aziz, et al., 2006.)
Accordingly, improved supply of organic phosphate, which not only supply soluble organic phosphates, but also release substantial amount of phosphorus through microbial mineralization of organically bound phosphorus, would be highly desirable.
II. Current Limitations with Production of Fertilizers and Inefficient Use.
Rock phosphate (RP) is a basic raw material used for manufacturing of chemical phosphatic fertilizer. Globally, there are four major types of phosphate resources in the world, including marine, igneous, metamorphic, and biogenic phosphate deposits which contain either of the fluor-apatite (Ca10(PO4)6F2), hydroxy-apatite (Ca10(PO4)6(OH)2), carbonate-hydroxy-apatite (Ca10(PO4CO3)6(OH)2), francolite, dahllite, and collophane compounds. (Straaten, 2002.) Reserves are primarily found in Northern Africa, China, the Middle East, United States, Brazil, Canada, Finland, Russia, and South Africa. Large phosphate resources have also been identified on the continental shelves and on seamounts in the Atlantic and the Pacific Ocean. World rock phosphate reserves are more than 300 billion tons, while annual consumption in 2011 was 191 million tons, up 20% from 2010.
The desired grade of rock phosphate for manufacturing classic chemical fertilizers is one having 30% P2O5 or higher, with low silica, magnesium, and other elements. Reserves of these grades are declining. A number of physiochemical processes are employed to improve P2O5 content of low-grade rock phosphate and to remove undesired elements. Physical and thermal up-gradation of rock phosphate is achieved through crushing and screening, scrubbing, de-sliming, flotation, and magnetic and gravitation separation. However, a substantial amount of energy is consumed and environmental pollutants are released in these processes.
Rock phosphate shows a considerable proportion of isomorphic substitution in the crystal lattice and has a variable proportion and amounts of accessory minerals and impurities. Research shows that rock phosphates are appropriate for direct use in acidic soils for the supply of available phosphorus, but are unsuitable for neutral to alkaline soils. (Sekhar and Aery, 2001.) Thus, the release of bioavailable phosphorus from insoluble phosphates in alkaline/calcareous soil is important for sustainable agriculture by mobilizing its phosphorus through a variety of advanced approaches where an inert phosphorus source is predictable, and can be rehabilitated into plant available form. (Kennedy and Smith, 1995; Caravaca, et al., 2004.)
In addition to phosphorous problems, depleted organic matter reserves of arid alkaline/calcareous soils further decrease crop production, as well as affect numerous soil metabolic processes. (Mullins, 2009.) Most organic matter decomposes quickly when applied in a hot arid climate, which explains why arid soils are poor in organic matter. But, organic matter is a universal remedy and is known to improve soil health and availability of nutrients to plant. Most organic wastes are a potential source of plant macronutrients as well as a large quantity of micronutrients. However, it is difficult to increase the organic matter content of soils that are well aerated, such as in coarse sands and soils in warm-hot and arid regions, because added materials decompose rapidly. (Hamza and Anderson, 2010.)
Generally, arid climate tends to enhance microbial decomposition of organic matter, and such soils are low in organic carbon. When any inoculum or microorganism cells are added to those soils, their population typically do not reach a level at which they can perform efficiently. In many cases, they do not survive long, often resulting in inconsistent performance. This situation alarmingly needs restoration of organic matter through exogenous application. But unlike chemical fertilizers, these organic amendments are not that rich in nutrients, particularly in phosphorus. Therefore, after the introduction of chemical fertilizers and high yielding varieties in cropping system, farmers are largely dependent on continuous injection of chemical fertilizers to meet high plant nutrient requirements.
But, the use of organic waste and chemical fertilizers not only requires constant replenishment (with significant resources to create), but they are also a source of pollution, requiring additional management for short term and long term environmental impacts. Organic waste management is a major environmental issue because constant population growth means commensurately more waste to be recycled. Likewise, chemical fertilizer manufacturing is known to generate a wide range of air emissions, hazardous materials, effluents, waste water, and other harmful byproducts are generated (e.g. hydrofluoric acid, silicon, tetrafluoride, fluoride, SO4 phospho-gypsum, NH4, NOx fluoride air, P2O5 effluents, dust fluoride effluents, chloride, cadmium, lead, radionuclides, and sulfur compounds, etc.). Further, chemical fertilizer manufacturing also consumes substantial energy, ranging from 120 to 450 KWh per ton of P2O5, depending on the process employed. Moreover, chemical fertilizer manufacturing consumes substantial amounts of water, ranging from 2 to 150 cubic meters per ton of P2O5, depending on the process employed. (UNEP Technical report, 1996; World Bank Group report, 2007.)
As the worldwide population grows, and the need for agricultural farming using organic waste and chemical fertilizers increase, recycling organic wastes is quickly becoming a major environmental issue. Composting organic residues is believed to be the best possible means to recycle. Using composted products improves soil properties, and in turn improves soil productivity, thus promoting the plant growth. (Vassilev and Vassileva, 2003.)
Thus, the efficient use of organic fertilizers is a key strategy not only for improving soil organic matter content and nutrients supply but also for reducing the input cost of mineral fertilizers and promoting healthier environment. (Bhattacharyya, et al., 2007; Ahmad, et al., 2007a.)
Organic approaches that partially supplement nutrients through organic sources (and which do not involve synthetic formulation) have gained considerable positive response during recent years. However, under the current hegemony of organic fertilizer practitioners, and because of accelerated decomposition, the use of organic materials remain poor in nutrient content and does not completely fulfill nutritional needs of crops, particularly phosphorus, for normal growth and yields. (Ahmad, et al., 2007b.)
However, lab research suggests that organic fertilizers can be used as rich carriers of plant growth promoting rhizobacteria that not only mobilize nutrients in soils but also facilitate nutrient uptake of less mobile nutrients, such as phosphorous, by altering root architecture. These synergistic effects benefit crops tremendously. (Shahroona, et al., 2008.) And novel plant growth promoting rhizobacteria (PGPR) isolates show promising attributes when developed and used as bio-fertilizers to enhance soil fertility and promote plant growth. (Dastgeer, 2010.)
However, a consistently lacking element in the use of PGPR isolates is the ability to consistently deliver the “right type” of bacteria that plays the appropriate role in phosphorus nutrition. Unless one consistently delivers the “right type” of bacteria that solubilize/mineralize inorganic and organic soils, such delivery cannot enhance phosphorous availability to plants. (Ahmad, et al., 2009; Walpola and Yoon, 2012.)
Based in part on the hegemony of current organic fertilizer users, and based on sparse research available on cultivating PGPR isolates, let alone those that are augmented with phosphate solubilizing microorganisms (PSM) or plant growth regulating microorganisms (PGRM) including, but not restricted to, prokaryotes such as algae, bacteria, protozoa, etc., and eukaryotes such as fungi, etc.; there exists a void in the industry as to the large scale production of bio-organo-phosphate (BOP) fertilizer using these technologies.
Accordingly, improved efficacy due to less probability of fixation, precipitation, or insolubilization than current commercial soluble chemical/inorganic fertilizers is desired. In addition, the environmentally conscious production, application, and management of organic fertilizers are likewise desirable. In sum, there exists a long-felt industry need for the large-scale production of bio-organo-phosphate (BOP) fertilizer that consistently and optimally delivers phosphorous to plants to help improve root architecture, enhance nutrient uptake, accelerate healthy growth, and hasten maturity.
III. Bioavailability of Immobile/less Mobile Nutrient Elements.
Agriculture has been providing food, feed, and shelter to global populations. Land use for agriculture is 38% of 13 billion hectares worldwide with 2.9% contribution to global GDP. According to 2020 projections, agriculture will be challenged to supply an increasing population, especially in developing countries. Fertilizers that contain both macro- and micronutrients are considered the most important input for sustainable crop production with an approximate increase of 25% in yield and productivity through their use. Both macro- and micronutrients play a critical role in overall agricultural productivity enhancement. Macronutrients of immobile/less mobile elements, such as phosphorus, and the majority of such micronutrients including, but not limited to, zinc, iron, boron, and manganese, are increasingly becoming a limiting factor in obtaining optimum crop productivity. It has been found that deficiency in some macronutrients and micronutrients affects the ability of plants to effectively uptake and use other mobile and immobile nutrients already present in the soil. In particular, zinc deficiency has recently been reported in a variety of crops. Zinc is also critical for human health. Its deficiency in humans may lead to compromised immunity, hair loss, skin lesions, loss of appetite, diarrhea, loss of taste and smell, and many other disorders. It is widely accepted that the best source of micronutrient supply to humans is through agricultural products. Therefore, micronutrient fortified crops, such as zinc-fortified crops, are an excellent source of nutritional balance for human beings.
Immobile/less mobile macro- and micronutrients are taken up by plants continuously from emergence to maturity. A variety of products containing immobile/less mobile nutrients are available in the market, however, application methods to target crops do not support sufficient availability of these nutrient elements to fulfill a plant's requirements throughout the entire crop cycle. For instance, uptake of zinc by crops is very low compared to the quantity applied. More than 90% of the applied zinc remains in non-available forms in the soil, particularly in alkaline soil, resulting in financial losses to farmers due to nutrient deficiency in plants and in lower yields. It has been documented that only 4-8% of the applied zinc is being utilized by the target crop in alkaline soils. Zinc deficiency also affects the plant's efficient use of other mobile and immobile/less mobile elements, even though these elements are sufficiently available in the soil, thus affecting agricultural productivity and the cost-benefit ratio of all nutrients for farmers.
A wide variety of commercial fertilizer products containing immobile/less mobile nutrient elements are available in the market. However, these either have low efficacy or are not suitable for all types of crops and soils. For instance, phosphorus use efficiency is around 20-25% and zinc use efficiency ranges between 4-8%. In addition, market availability of products including zinc and other micronutrients in a form readily available to crops is yet another crucial factor that is seriously affecting farmers' ability to enhance productivity.
Accordingly, there is a need to develop an effective fertilizer to enhance bioavailability of immobile/less mobile macro- and micronutrients in order to reduce losses and improve agricultural productivity.